TechArchive

Original article date: October 1998

An apparently impossible combination? G PLATT of Centa Transmissions explains how the benefits of cycloidal and planetary gears can be combined in a single unit – and why.

Cycloidal planetary (epicyclic) and spiral bevel gears all have their own operating characteristics and may be used individually or in combination depending on the application. Each gear principle is available in a standard industrial form or as a precision servo drive.

The spiral bevel system is a right-angle drive offering low ratios high efficiencies and high input speeds. The gearbox can be used independently or in conjunction with either the cycloidal or planetary gearboxes to give a high ratio right-angle drive. Low backlash versions of the spiral bevel gearbox are available.

The cycloidal system is coaxial and offers high reduction ratios in a single stage. It is suitable for heavy duty applications having a very high shock load capability of up to 500%. Whilst multiple contact torque distribution assists overload capacity the ability of cycloidal drives is exceptional since they will handle instantaneous peak shock overloads of up to 500% without risk of unit failure. Zero backlash versions are available for precision drives.

The planetary system is also coaxial and is particularly suitable for high torque low speed applications. it is extremely price competitive against other gear systems and offers high efficiency in a very compact size. For similar output torques the planetary gear system is the most compact gearbox on the market. Again versions of this system are available with low backlash down to less than 0.5 arc min.

Planetary units share the load torque equally between three or more planet gears in each reduction stage. The multi-mesh balanced reaction forces eliminate the high radial separating forces of conventional gearing. Similarly the multiple contact roller and twin or triple disc configuration of a cycloidal drive distributes the reaction load evenly around the outer circumference of the unit.

Distributed torque and coaxial configuration ensures that both planetary and cycloidal drives have extremely compact dimensions – axially and diametrically – and are lighter than helical or worm gear units for a given ration and output torque. Both types will operate in either direction of rotation and can also be used as speed increasing units.

Efficiencies in excess of 94% for cycloidal units and more than 97% for planetary units are achieved on single reduction stages. Output torques start at 0.2Nm and range up to 60 000Nm for cycloidal drives whilst planetary gears handle up to 550 0

Depending upon type and size planetary units offer single stage reduction from 3:1 to 10:1 with multi-stage ratios is excess of 10 000:1. By comparison cycloidal units offer ratios from 6:1 to 179:1 with multi-stage ratios in excess of 10 000 0

High torsional stiffness and low input inertia are inherent characteristics of planetary and cycloidal systems making both eminently suitable for highly dynamic high frequency indexing applications which require rapid response to acceleration deceleration and reversing torques. Both versions are combined in a special unit designed by Centa Tranmissions for positioning/indexing applications in paper converting and similar industries.

The new design combines the balanced torque reaction forces of planetary gears and their ability to accept modern high speed servomotor inputs with the high torsional stiffness high torque and high overload capacity of a cycloidal speed reducer. The result is an extremely compact highly dynamic coaxial or right-angle drive unit.

The combined reduction stages give ratios from 87:1 to 100 000:1. An exceptional torque multiplication factor is achieved – output torque capacity ranging from 180 to 5500Nm with an occasional peak overload ability of 24 000Nm as the input/output shaft diameters clearly demonstrate.

• Centa Transmissions
• 01274 531034

October 1998